Alternate shuttle vector plasmid systems capable of replicating both in bacteria and animal cells could provide immediate instant expression of cDNAs cloned in such cells. By inserting cDNA within the appropriate region, it is possible to express exogenous genes in monkey cells stably transformed by SV40 (cos cells); these cells are permissive for replication of recombinant SV40-plasmids. We are in the process of constructing a cDNA library in the above system using as templates, mRNA from cells infected with RS virus or parainfluenza 3 virus. Cells were treated with high doses of actinomycin D during infection to reduce the level of cell specific mRNAs. These recombinant plasmids in groups will be used to transfect monkey (cos) cells and the in vitro translational products will be identified by a variety of immunological methods.